1. Field of the Invention
This invention relates to a stimulable phosphor and a radiation image storage panel utilizing the same, and more particularly relates to a rare earth element activated complex halide stimulable phosphor and a radiation image storage panel having a fluorescent layer comprising the same.
2. Description of the Prior Art
As is well known in the art, a photographic method using a silver salt, e.g. radiography, in which an X-ray film having an emulsion layer comprising a silver salt is used in combination with an intensifying screen has generally been employed to obtain a radiation image. Recently, from the viewpoint of problems such as the shortage of silver resources, a method of obtaining a radiation image without using a silver salt has been desired.
An example of such a method is disclosed in U.S. Pat. No. 3,859,527. In the method of the patent, there is used a radiation image storage panel comprising a stimulable phosphor which emits light when stimulated by visible light or infrared rays after exposure to a radiation (The term "radiation" as used herein means an electromagnetic wave or a corpuscular radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, high-energy neutron rays, cathode rays, vacuum ultraviolet rays, ultraviolet rays, or the like.). The method comprises the steps of (i) causing the stimulable phosphor of the panel to absorb a radiation passing through an object, (ii) scanning the panel with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to sequentially release the radiation energy stored in the stimulable phosphor of the panel as light emission, and (iii) electrically converting the emitted light into an image.
The radiation image storage panel employed in the above-mentioned method for recording and reproducing a radiation image has at least a fluorescent layer comprising a proper binder and a stimulable phosphor dispersed therein. Although the fluorescent layer itself can be a radiation image storage panel when the fluorescent layer is self-supporting, the fluorescent layer is generally provided on a proper substrate to form a radiation image storage panel. Further, a protective layer for physically and chemically protecting the fluorescent layer is usually provided on the surface of the fluorescent layer intended for exposure. Furthermore, a primer layer is sometimes provided between the fluorescent layer and the substrate to closely bond the fluorescent layer to the substrate. From the viewpoint of practical use of the radiation image storage panel, it is required for the panel to have high sensitivity. Further, it is required for the panel to have high ability to store the radiation energy absorbed thereby, that is, to exhibit little fading. Accordingly, it is required for the stimulable phosphor employed in the radiation image storage panel to emit light of high luminance upon stimulation thereof and to exhibit little fading.
As a stimulable phosphor which can be employed in the above-mentioned radiation image storage panel, a cerium and samarium activated strontium sulfide phosphor (SrS:Ce, Sm), a europium and samarium activated strontium sulfide phosphor (SrS:Eu, Sm), a europium and samarium activated lanthanum oxysulfide phosphor (La.sub.2 O.sub.2 S:Eu, Sm) and a manganese and halogen activated zinc cadmium sulfide phosphor [(Zn,Cd)S: Mn, X, wherein X is a halogen] are known (see the above-mentioned U.S. Pat. No. 3,859,527). However, the luminance of light emitted by these stimulable phosphors upon stimulation thereof is exceedingly low. Further, since the wavelength ranges of the stimulating rays of these stimulable phosphors lie mainly in the infrared region, the traps for storing radiation energy of these stimulable phosphors are relatively shallow, and therefore, these stimulable phosphors exhibit very high fading. Accordingly, the utility of the radiation image storage panels in which these stimulable phosphors are employed is low.
U.S. Pat. No. 4,239,968 discloses the use in a radiation image storage panel of a rare earth element activated barium flourohalide stimulable phosphor represented by the following formula: EQU BaFX:yA
wherein X is at least one halogen selected from the group consisting of Cl, Br and I, A is at least one rare earth element selected from the group consisting of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb and Er, and y is a number satisfying the condition of 0.ltoreq.y.ltoreq.0.2. The rare earth element activated barium fluorohalide phosphor emits light of much higher luminance than the stimulable phosphors described in the above-mentioned U.S. Pat. No. 3,859,527 upon stimulation thereof. Further, because the wavelength range of the stimulating rays of the rare earth element activated barium fluorohalide phosphor is from 500 to 1100 nm, and the optimum wavelength range thereof is from 500 to 800 nm, the trap for storing radiation energy of the rare earth element activated barium fluorohalide phosphor are deeper than those of the stimulable phosphors described in the above-mentioned U.S. Pat. No. 3,859,527, and therefore, the former exhibits less fading than the latter. Accordingly, the radiation image storage panel utilizing the above-mentioned rare earth element activated barium fluorohalide phosphor has much higher sensitivity and exhibits less fading than the radiation image storage panels utilizing the stimulable phosphors described in the above-mentioned U.S. Pat. No. 3,859,527. Although, as mentioned above, the utility of the radiation image storage panel utilizing the above-mentioned rare earth element activated barium fluorohalide phosphor is higher than that of the radiation image storage panels utilizing the stimulable phosphors described in the above-mentioned U.S. Pat. No. 3,859,527, further improvement of the sensitivity and the fading property thereof is desired.